Image: Comparative sizes
R. Hurt / Caltech / JPL / NASA
An artist's conception shows the comparative scale of Earth, its moon, Pluto, Sedna and a smaller object on the solar system's fringe, Quaoar.
By Senior Science Writer
updated 3/16/2004 5:37:53 PM ET 2004-03-16T22:37:53

Our corner of the galaxy got a little stranger this week with the discovery of Sedna, the most distant object ever spotted in the solar system. Now astronomers are puzzling over how it got there.

The most intriguing idea is that there might be another world as big as Earth, a gravitational bully lurking in some unexplored corner of the solar system.

Here's the problem: Scientists can't figure out how Sedna, which is about three-fourths as big as Pluto, came to have such a strange orbit around the sun. Sedna's path is highly elliptic. It ranges from 76 astronomical units, or AU, when it is closest to the sun to 1,000 AU when it is farthest. One AU is the distance from Earth to the sun, about 93 million miles or 150 million kilometers.

"How on Earth could anything get into an orbit like that," wonders astronomer Brian Marsden. He suggests another sort of Earth might have had something to do with putting Sedna on its current, odd course.

Snowball Earth twin?
Michael Brown, the astronomer at California Institute of Technology who led the discovery of Sedna, said the most likely scenario involves the sun having been born in a star cluster, and several stars that were then closer to the solar system — still more than 10,000 AU away — were responsible for ejecting objects like Sedna.

Several astronomers not involved in the discovery support the idea that Sedna was lured outward by a star. But others don't buy that explanation.

"I don't really like that," said Marsden, who heads the Minor Planet Center in Cambridge, Mass., where newfound solar system bodies are cataloged.

Marsden favors an object closer in, a "planetary object," he told, perhaps at between 400 and 1,000 AU.

"Perhaps there's more than one planet out there," Marsden said. "Who knows? But let's suppose it is something of an Earth mass, maybe even a few Earth masses. A close approach could throw this object [Sedna] from something more circular into something more eccentric."

Marsden says such a scenario leaves open the question of how an Earth-sized planet could have formed so far from the sun, where raw material should have been sparse, according to current theory.

Brown said an Earth-sized planet is indeed a possibility. But his team's calculations put it at about 70 AU.

"We think it's unlikely, because we think we would have found it by now," Brown said in a telephone interview.

Alan Boss, a planet-formation theorist at the Carnegie Institute in Washington, agrees that a passing star or dense cloud of gas is the more likely cause for Sedna's strange travels. Boss said it would be "hard to imagine" forming an Earth-sized object out where the interaction would have taken place.

Region to explore
But Brown said there is one unexplored region of space left, amounting to about 20 percent of the sky, that hasn't been searched for an Earth-sized object that would be orbiting at 70 AU and presumably in the main plane of the solar system. It is the region toward the bright galactic center, which is harder to search.

Brown said his team is considering making that search now.

If Marsden's idea is on track, and there is an Earth-sized planet several hundred AU away, it would easily have escaped detection by current surveys.

Either way, this wouldn't involve any ordinary Earth. Any object beyond Pluto would be frozen solid and would not be a strong candidate for bearing life.

In a sideline argument, astronomers have differing opinions on whether Sedna should be considered a planet. Many argue that Pluto should never have been called a planet, because it is more like Sedna and other objects beyond Neptune — small and with offbeat orbits.

In fact, however, there is no astronomical definition for the term "planet." But the International Astronomical Union has made it clear in recent years that Pluto, despite how most astronomers now view it, will not be stripped of its planethood. It is not known what astronomers would call an Earth-sized object found beyond Neptune in a circular orbit, but it would be hard not to consider it a planet.

Earths even farther out?
Alan Stern of the Southwest Research Institute adds another twist to the whole puzzle. Stern thinks there could be Earth-sized planets in the Oort Cloud, the most distant region of the solar system.

Image: Oort Cloud
This diagram, produced by NASA's Jet Propulsion Laboratory, shows the nine planets as a small inset within the much larger Oort Cloud, extending trillions of miles out from the sun.
Brown's team said they thought Sedna should be counted as the first known object of the otherwise theoretical Oort Cloud. The distant reservoir of small icy objects is thought to exist based on the orbits of some comets that zoom through the inner solar system now and then, and then disappear into deep space.

Nobody knows what's actually in the Oort Cloud, however.

"I would say that is likely," Stern said in regards to possible Earth-sized planets in the Oort Cloud. In the early years of the solar system, he explained, objects as massive as Earth are thought to have hit Uranus and Neptune. Computer simulations show most of the hypothetical Earth-mass objects "would be ejected from the outer-planets region, not accumulated in Uranus and Neptune, so we could someday find these frozen relics in the Oort Cloud."

Ferreting out worlds that far away would be a monumental challenge. The Oort Cloud is said to stretch nearly halfway to the next known star.

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